I have never read any GGK. What one book should I request from the library to see if I like him?
Not the easiest question, really, since GGK's books seem to fall into three "phases" thus far. I say "seem to" because it's not clear yet as to whether Ysabel represents a definite shift in his career, or if it's a one-off.
So far, GGK has only produced one work of traditional "high fantasy"; that is the Fionavar Tapestry trilogy. If one is most comfortable with fantasy of that nature, then that's the place to start. Fionavar does take a little while to get going, but there's a lot of poetry in the prose, and it does get moving eventually -- especially by Chapter Nine. However, if one is looking for a single book and doesn't want to commit to an entire trilogy, then look to the historical fantasies. (Although it's not as if Fionavar is terribly massive by contemporary standards; the entire thing is about as long as The Lord of the Rings, which is, as most readers of contemporary fantasy are aware, dwarfed nowadays by the massive doorstops that constitute high fantasy now.)
But after that begins his "historical fantasy" phase, when he would use each book to create a fictional world based on some real historical locale, so as to explore various historical themes without having the constraints of real historical events. This phase begins with Tigana and possibly ends with The Last Light of the Sun.
Now, each of these books contains small touches -- "grace notes", as GGK calls them -- which reference Fionavar, but none of these books assumes a previous reading of that trilogy. Also, The Lions of Al-Rassan, the Sarantine Mosaic duology, and Last Light are all set in the same geographical world, but none of these books assume a reading of the previous ones, although there are again "grace notes" which those who have read the previous works in that world will understand. These books and their historical analogs are as follows:
Tigana: Renaissance Italy
A Song for Arbonne: Provence in the time of the Albigensian Crusade
The Lions of Al-Rassan: Spain at the time Ferdinand and Isabella drove out the Moors
The Sarantine Mosaic: Byzantium at the time of Emperor Justinian I
The Last Light of the Sun: Britain at the time of Alfred the Great
These historical parallels are very broadly portrayed, so readers deeply familiar with any of these time periods coming to GGK's book in that same general timeline may find themselves frustrated if they don't understand what GGK is actually doing. So which of these to start with? I'd recommend either Tigana or Arbonne, actually. Lions wouldn't be a bad starting point either, although since it's my favorite I tend to lean toward saving the best for last. The Sarantine Mosaic is in two volumes, so if one wants a single book, look out for that, although again it's not like we're talking about a level of commitment like one of George RR Martin's tomes. The prologue of Sailing to Sarantium is a bit difficult sledding, I admit.
I don't think I'd recommend Last Light as a starting point, for several reasons. First, it does seem, if I remember correctly, to assume a greater familiarity with GGK's fantasy Europe than the others set in that same world; second -- and I'll flog this point more when I re-read the book -- it has no map, which I think is a much bigger flaw than GGK himself seemed to when I reviewed the book; third, its emotional tone is cooler than his earlier works. He had reason for this, and it works, but the book does stand as something a bit separate, on that basis.
That leaves Ysabel. I'm a bit conflicted on whether it's a good starting point for GGK or not. It is, in part, a direct sequel to Fionavar, or at least a follow-up; but it's set in our world and not Fionavar, so I'm not sure how well it works for people who haven't read the Tapestry. It's a "magic in our world" story, which puts it more in line with the kind of thing Charles de Lint writes than GGK's previous output. As of this writing, no one knows what GGK's next book is going to be, so we don't know if he's moving in this "real world fantasy" direction or if he's going back to his fantasy Europe or somewhere else. (I've long hoped he'd do a "historical China" novel, as long as he was doing his "historical fantasy" thing, but we'll see.) I think one could start with Ysabel, with the warning that it does represent something fairly different from GGK's previous output.
Boiling down: I'd either start with Fionavar, Tigana, or Arbonne.